Chemical vapor deposition (CVD) is widely used in the semiconductor industry for manufacturing integrated circuits. There are a variety of CVD methods, such as atomic layer deposition (ALD), metal organic CVD (MOCVD), plasma enhanced CVD (PECVD), and the like. Although these methods may be based on different mechanisms, they all utilize chemical reactions, wherein gaseous reactants are introduced into reaction chambers, in which the chemical reaction occurs and products of the chemical reactions are deposited.
With the increasing down-scaling of integrated circuits, devices are becoming increasingly smaller, and accordingly, the components are decreasing in size in each of the devices. For example, the thicknesses of gate dielectrics of transistors will be scaled down by about two angstroms in next generation. Currently, the thickness of conventional SiO2 gate dielectrics is close to its theoretic limit. Dielectric materials with high dielectric constants (high-k) are thus used to replace SiO2.
High-k dielectric materials, such as HfSiOx and HfO2, are often formed using ALD. As opposed to most of the other CVD methods, which are characterized by continuous deposition and concurrent flow of precursors, ALD is based on the sequential growth of individual mono-layers or fractions of a mono-layer in a well-controlled manner. In ALD, the growth surface is alternately exposed to only one of several chemicals. For example, individual precursors are supplied to the reaction chamber one at a time. The exposure steps are separated by inert gas purge steps in order to remove any residual chemically active source gas or by-products before another precursor is introduced into the reactor. Thus, ALD consists of repetitions of individual growth cycles. During each exposure step, precursor molecules react with the surface of the wafer in the reaction chamber until all available surface terminals are reacted. Precursor chemistries and process conditions are chosen such that no further reaction takes place once reactions on the surface are completely saturated. Surface saturation guarantees the self-limiting nature of ALD. Precursors are preferably overdosed so that the reactions are independent of potential variations in the amount of precursors supplied to the surface. Thus, surface chemistry, rather than a precise control of tool specific process parameters such as precursor flow and partial pressure, governs the film growth. A known and constant thickness is deposited per growth cycle.
ALD has the ability of forming high-quality films, and is particularly useful for forming thin films. However, when the physical thicknesses of gate dielectric layers reach about 20 Å or less, the leakage of gate dielectric layers becomes a great concern, and thus the quality of gate dielectric layers requires further improvement. Conventionally, ALD has a relatively long incubation time, potentially causing the film quality to be adversely affected. In addition, due to the low temperature of ALD, incomplete reactions may occur, which also affects the quality of films. Accordingly, new methods are needed to further improve the quality of the films.